Interferometric Synthetic Aperture Radar (InSAR) is used to obtain high spatial resolution surface deformation maps. InSAR measurements are obtained through the post-processing of active spaceborne and airborne microwave-based imagery. While the ultimate mapping resolution of an InSAR product is impressive (30 meter pixel size for ENVISAT or ERS and less than 5 meters pixel size with RADARSAT-2), the true power of InSAR resides in its tremendous spatial coverage. A single synthetic aperture radar image has the ability to cover a roughly 100 km by 100 km area.
One limitation of InSAR is that it only provides land surface deformation information in the line of sight of the spacecraft or aircraft taking the image. This is also referred to as the “slant-range” perspective of the radar sensor. The effect of this is that InSAR technology is only able to detect motion in this one slant-range axis. This is incompatible with many surface deformation environments, which frequently manifest themselves in three axes. InSAR based slant-range motion observations result in a loss of valuable information about the true surface motion. Another shortcoming of InSAR is that it lacks an absolute reference, because it provides only the change in a surface from one time period to another. Similar to analysis of elevation using tilt measurements, this can result in a build-up of errors over time. Furthermore, within an analysis, the choice of what constitutes zero motion from one time period to another is subject to errors due to satellite positioning and atmospheric conditions. As a result, an image generated from InSAR alone is best used to gauge only relative motion between points in the image, rather than the absolute motion of any single point.